Methods for combining components of varying stages of cure

ABSTRACT

The present invention provides a method of fabricating a composite structure from components of varying stages of cure while reducing the steps associated with the fabrication process of infusing, curing, and bonding composite materials to form a hybrid unitized structure. The method provides a pre-cured stiffener and a pi-preform having a clevis and a base portion. The pre-cured stiffener may be inserted into the clevis of the pi-preform to form a composite structure assembly. The composite structure assembly may be infused with a resin system at the time of cure and bonded to a second composite structure to form a hybrid unitized structure.

TECHNICAL FIELD

The present invention relates generally to composite materials. Moreparticularly, the invention relates to methods for combining componentsof varying stages of cure and for creating unitized hybrid compositestructures.

BACKGROUND INFORMATION

Over recent years, composite materials have become an increasinglydesirable material for aircraft structures. Composite materialstypically comprise strands of fibers (e.g., glass- and/or carbon-fiber)mixed with a resin. For example, many commercially produced compositesuse a polymer matrix material as the resin. In fact, there are manydifferent polymers available, depending upon the starting rawingredients. The more common polymer may include, for example,polyester, vinyl ester, epoxy, phenolic, polyimide, polyamide,polypropylene, and PEEK. During fabrication, fibers may be often wound,or woven, into a sheet of material and then impregnated (e.g., infused)with a resin. Once the fibers have been impregnated with a resin, thecomposite material may then be formed into the desired shape and cureduntil properly hardened.

Composite materials have an advantage of being extremely lightweight andhaving high strength. As a result, they are useful in, among otherthings, aircraft applications. Additionally, composite structures may bemolded into desired shapes and configurations, thus eliminating the timeand cost associated with fabricating shapes using traditional methodsand materials. While many parts manufactured using composite materialscould also be made from metal, a metallic part of the same strength andstiffness would be significantly heavier.

An example composite material is fiberglass, which consists of a matrixof glass-fiber, impregnated with a polymer resin. The glass-fiberprovides tensile strength, but is flexible (like cloth). To providerigidity, resin is used to lock the glass-fibers in place, thusresulting in a strong, relatively lightweight material that may be cut,drilled and otherwise manipulated while being resistant to moisture andchemicals.

An example group of composite materials includes carbon-basedcomposites, which are strong and light fiber-reinforced polymers thatcontain carbon-fibers instead of glass-fiber. Examples of suchcarbon-based composites include carbon-fiber-reinforced polymers orcarbon-fiber-reinforced plastics (CFRP or CRP). The polymer used to lockthe carbon-fiber in place is typically an epoxy, but other polymers,such as polyester, vinyl ester or nylon, are sometimes used.Carbon-based composites may also contain fibers such as, for example,para-aramid synthetic fiber-reinforced polymers (e.g., Kevlar®), nickel,titanium, glass-fiber, as well as carbon-fiber and carbon nanotubes. Dueto their strength and lightweight construction, carbon composites havemany applications in both the aerospace and automotive fields.

Another example composite material is Glass Laminate Aluminum ReinforcedEpoxy (GLARE). GLARE typically comprises several thin layers of aluminuminterspersed with layers of glass-fiber “pre-preg” (i.e.,“pre-impregnated” composite fibers where a material, such as epoxy isalready present), bonded together with a matrix such as epoxy.Initially, pre-preg is flexible and sticky, but becomes hard and stiffonce it has been heated (i.e., during the curing process). AlthoughGLARE utilizes standard metallic materials such as aluminum, itsmanufacturing process, inspection and repair are more representative ofother composite materials.

However, as recognized by U.S. Pat. No. 7,681,835 to Simpson et al., adrawback to certain composite materials is the actual assembly, orjoining, of the composite materials. Unlike more traditional materials(e.g., metals), different considerations must be made for assemblingcomposite materials. For example, placing holes in composite materialsfor attachment of fasteners severs the strands of fibers within thematerial and creates weak points within the material. While formingholes in the composite material by displacing the strands of the uncuredfibers prevents severing of the fibers, this process is time-consumingand often impractical. Another alternative for assembling compositematerials is the use of high-strength epoxies. Epoxies have an advantageof limiting the number of manufacturing steps. However, the distributionof the epoxy and the placement of the parts together can requireexpensive machines and numerous jigs (e.g., tooling). Moreover, suchstructures routinely involve multiple sets of tools, are very laborintensive, require several cure cycles and can require B-staged materialwith set expiration dates.

Therefore, there is a need in the art, for an improved method ofcombining, or joining, composite components of varying stages of curethat alleviates the aforementioned drawbacks.

SUMMARY

The present disclosure endeavors to provide methods for creating aunitized hybrid composite structure. The present disclosure alsoendeavors to provide a system and method for combining compositecomponents of varying stages of cure and components of either similar ordissimilar materials.

According to a first aspect, a method of fabricating a compositestructure from components of varying stages of cure comprises the stepsof: providing a stiffener component; providing a dry fabric component;combining the stiffener component and the dry fabric component to forman assembly, wherein the stiffener component provides structural supportto the assembly; infusing the assembly with resin to yield an infusedassembly; and curing the infused assembly to yield a cured assembly.

According to a second aspect, a method of fabricating a compositestructure from components of varying stages of cure comprises the stepsof: providing a pre-cured composite stiffener; providing a dry fabriccomponent having a base portion and a clevis configured to receive thepre-cured composite stiffener, wherein the clevis is substantiallyperpendicular to the base portion; wrapping a first resin film around afirst edge of said pre-cured stiffener; inserting the first edge of saidpre-cured composite stiffener into the clevis of the dry fabriccomponent to form a composite structure assembly; applying a secondresin film to a surface of the dry fabric component; securing thepre-cured stiffener substantially perpendicular to the base portion ofthe dry fabric component, wherein the pre-cured stiffener providesstructural support to the composite structure assembly; infusing thecomposite structure assembly; and curing the composite structureassembly.

According to a third aspect, a method of fabricating a compositestructure from components of varying stages of cure comprises the stepsof: providing a pre-cured composite stiffener; providing a dry fabriccomponent having a base portion and a clevis configured to receive thepre-cured composite stiffener, wherein the clevis is substantiallyperpendicular to the base portion; wrapping a first resin film around afirst edge of said pre-cured stiffener; inserting the first edge of saidpre-cured composite stiffener into the clevis of the dry fabriccomponent to form a composite structure assembly; applying a secondresin film to a surface of the dry fabric component; securing thepre-cured stiffener substantially perpendicular to the base portion ofthe dry fabric component, wherein the pre-cured stiffener providesstructural support to the composite structure assembly; infusing thecomposite structure assembly by heating the assembly to a firsttemperature for a first period of time; and curing the compositestructure assembly by increasing the heat to a second temperature,wherein the second temperature is greater than the first temperature.

According to a fourth aspect, a method of fabricating a compositestructure from components of varying stages of cure comprises the stepsof: providing a pre-cured composite stiffener; providing a dry fabriccomponent having a base portion and a clevis configured to receive thepre-cured composite stiffener, wherein the clevis is substantiallyperpendicular to the base portion; wrapping a first film adhesive arounda first edge of said pre-cured stiffener; wrapping a first resin filmaround the first edge of said pre-cured stiffener; inserting the firstedge of said pre-cured composite stiffener into the clevis of the dryfabric component to form a composite structure assembly; applying asecond film adhesive to a surface of the dry fabric component; applyinga second resin film to the base portion of the dry fabric component;securing the pre-cured stiffener substantially perpendicular to the baseportion of the dry fabric component, wherein the pre-cured stiffenerprovides structural support to the composite structure assembly;applying a film adhesive to the exterior base portion of the dry fabriccomponent securing the pre-cured stiffener with dry fabric component toanother substrate to which it will bond; infusing the compositestructure assembly by heating the assembly to a first temperature for afirst period of time; and curing the composite structure assembly byincreasing the heat to a second temperature, wherein the secondtemperature is greater than the first temperature.

In certain aspects, the method may further comprise the step of bondingthe cured assembly with a component to form a hybrid unitized structure.

In certain aspects, the stiffener component is fabricated from acomposite material, a carbon-fiber composite material and/or metal.

In certain aspects, the dry fabric component may be a three-dimensional,woven pi-preform, wherein said pi-preform contains a clevis configuredto receive an edge of the stiffener component.

In certain aspects, the method may further comprise the steps of: (i)wrapping a resin film around a first edge of said stiffener component;and (ii) inserting the first edge of said stiffener component into theclevis of said pi-preform.

In certain aspects, the method may further comprise the steps of: (i)wrapping a resin film around a second edge of said stiffener component;and (ii) inserting the second edge of said stiffener component into theclevis of the second pi-preform.

In certain aspects, the component comprises Glass Laminate AluminumReinforced Epoxy.

In certain aspects, the dry fabric component comprises at least one of:(1) glass-fiber; (2) carbon-fiber; or (3) para-aramid synthetic fiber.

In certain aspects, the stiffener component is (i) L-shaped or (ii)J-shaped.

In certain aspects, (i) the first temperature is between 150 and 200degrees Fahrenheit, (ii) the second temperature is between 200 and 300degrees Fahrenheit, (iii) the first period of time is between 10 and 25minutes, and (iv) the second period of time is between 240 and 360minutes.

In certain aspects, (i) the first temperature is about 175 degreesFahrenheit, (ii) the second temperature is about 250 degrees Fahrenheit,(iii) the first period of time is between 15 and 20 minutes, and (iv)the second period of time is between 290 and 310 minutes.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other advantages of the present invention will be readilyunderstood with reference to the following specifications and attacheddrawings wherein:

FIG. 1 illustrates a cross sectional view of an L-shaped stiffener witha pi-preform sandwiched between two different mandrel halves;

FIGS. 2 a through 1 i illustrate a first example infusion and cureprocess using a pre-cured stiffener and a pi-preform;

FIGS. 3 a through 3 f illustrate a second example infusion and cureprocess using a pre-cured stiffener and a pi-preform;

FIGS. 4 a through 4 h illustrate cross-sectional views of examplestiffener shapes and configurations;

FIG. 5 illustrates a first example unitized hybrid structure wherein aplurality of pi-preform stiffener assemblies are used to providerigidity and strength;

FIGS. 6 a through 6 c illustrate a second example unitized hybridstructure;

FIGS. 7 a and 7 b illustrate a third example unitized hybrid structure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will be describedhereinbelow with reference to the accompanying drawings. In thefollowing description, certain well-known functions or constructions arenot described in detail since they would obscure the invention inunnecessary detail. For this application, the following terms anddefinitions shall apply:

As used herein, the term “composite material” refers to material madefrom two or more constituent materials with different physical orchemical properties, including resin-fiber composites. Examples of suchcomposite materials include fiberglass, carbon-fiber-reinforced polymers(“carbon-fiber”), Glass Laminate Aluminum Reinforced Epoxies (GLARE),para-aramid synthetic fiber-reinforced polymers (e.g., Kevlar®) and anyother composite material known in the art of manufacturing aircraft,watercraft and land craft.

As used herein, the term “composite component” refers to an articlefabricated from a composite material.

As used herein, the terms “cure” and “curing” refer to the process oftransforming an initially liquid resin into its final rigid solid form.

As used herein, the terms “bond” and “bonding” refer to the process ofjoining two or more components, including, but not limited to, compositecomponents.

As disclosed herein, it is an objective of the present invention toprovide systems and methods for combining non-composite componentsand/or composite components of varying stages of cure. That is,structures that would normally require several cure steps or independentfabrications may instead be more efficiently fabricated. A consolidatedinfusion and cure cycle streamline the fabrication process by mergingmultiple steps into a single step, thereby reducing tooling, fabricationtime, complexity of tools, oven run time, and potential contamination ofindividual components by consolidating processes. Moreover, aconsolidated infusion and cure cycle extends the shelf life ofcomponents (e.g., expensive dry fiber components, such asthree-dimensional woven pi-preforms) by not infusing (i.e.,impregnating) them with resin until the time of cure, thus reducingoverall costs and preventing damage to B-staged parts (e.g., parts thathave undergone a partial cure).

Accordingly, a dry fiber component, such as a dry, three-dimensionalwoven fiberglass pi-preform, can be infused while simultaneously curingand bonding said dry pi-preform to other structures, such as previouslycured composite structures. While a dry three-dimensional wovenfiberglass pi-preform is described and illustrated, numerous other dryfiber components may be used, which may be of varying shapes and sizes,such as multi-spoked three-dimensional woven shapes (e.g., havingmultiple devises).

Previously cured structures, which include, but are not limited to,pre-cured stiffeners, can also function as tooling, thus eliminating thetime and expense attributed to the additional tooling steps that wouldnormally be required by the process. While a pre-cured carbon-fiberstiffener is described and illustrated in the various figures, othermaterials are contemplated and may be used in lieu of, or in conjunctionwith, carbon-fiber. Such materials include, for example, polymers, othercomposites, metals, plastics, nano-materials, and ceramics.

Moreover, using a dry fiber component, such as a dry three-dimensionalwoven fiberglass pi-preform, provides the additional function ofcreating a barrier between the stiffener and composite structure. Forexample, a glass-fiber pi-preform may provide a barrier between analuminum GLARE component and a carbon-fiber stiffener, thus reducing, oreliminating, the risk of corrosion that can result when such materialsmake contact with each other. Specifically, in cases where GLARE and acarbon-fiber are used, the barrier deters galvanic corrosion. Similarly,a glass-fiber pi-preform may act as an insulator and/or isolator betweenthe carbon fiber and aluminum surface of the GLARE. More specifically,in cases were electrical power and/or signals may be carried through theconductive carbon fiber structure, the fiber-glass prevents shorting tothe aluminum of the GLARE by acting as an electrical insulator and/orisolator.

Using prior methods, such infusion processes would have required aseparate set of tooling to: (i) orient and retain the pi-preforms; and(ii) allow sufficient saturation of the fibers without destroying thegeometry. This tooling is often used only for infusion and is thereforeunnecessary after the infusion process is complete. Indeed, using priorprocesses to manufacture a component having a glass-fiber pi-preform andcarbon-fiber stiffener would have required substantially more steps,time and expense. For example, a single-piece pi-preform withcarbon-fiber stiffener would have been made by (1) curing thecarbon-fiber stiffener, (2) infusing the pi-preform, (3) curing thepi-preform, and (4) then finally bonding the carbon-fiber stiffener tothe glass-fiber pi-preform. Moreover, during that process, uniquetooling would have been required to infuse the pi-preform. Furthermore,after infusion, infused preforms (e.g., pi-preforms) would be muchharder to handle due to the added stiffness and tackiness of the resin.Finally, infusion was typically done per order which also meant thatpi-preforms were delivered with an expiration date based on the resin.

The present system and method overcomes these deficiencies byconsolidating several of the manufacturing processes to increase thespeed and efficiency of fabrication while reducing the necessary toolingrequired for the finished product. For example, a carbon-fiber stiffenermay be pre-cured and configured to function as a backbone and/or supportduring a consolidated infusion, cure and bond process. With theassistance of mandrels that can be used during other phases ofmanufacturing, pre-cured carbon-fiber stiffeners may be bonded with adry fiber component (e.g., a pi-preform) to function as part of thetooling used during infusion, thereby providing a seamless transitionfrom infusion to cure. In essence, the present method effectivelyconsolidates what would normally require at least two curing steps, oneof which includes a separate infusion step, and a bonding step into onlytwo curing steps, therefore eliminating at least one iteration ofinfusion and tooling, while also decreasing the overall fabrication timeand cost. It is generally advantageous to eliminate these moretraditional secondary bonding steps because they require more prep workon components before assembly as well as more tolerance on individualparts/tools for assembly and/or differing adhesives, which couldnegatively affect the final properties of the unitized structure. Inaddition, simultaneously curing a multi element component as one unitreduces the amount of extra/scrap material that often results whencomponents are separately cured and bonded/assembled. For example, usingprior techniques, bonding two cured components would likely require thatthe components be trimmed to the exact shape needed before usingadditional tooling to jig the components into place and then bond themtogether. This step is omitted using a consolidated infusion and curecycle as disclosed herein.

In the consolidated infusion and cure cycle, resin film may be applieddirectly to the dry fiber component and stiffener. With the assistanceof a mandrel, additional pressure may be provided during theconsolidated infusion and cure cycle of the component fabrication. Bycontrolling the oven temperature, the consolidated infusion and curecycle allows for resin to flow into the dry fiber component(infusion/impregnation) and then cures the impregnated fiber componentby increasing the temperature while simultaneously bonding theimpregnated fiber component with the stiffener. This consolidatedinfusion and cure cycle saves time (e.g., requiring fewer cure cyclesand fabrication steps) and reduces tooling (e.g., eliminates infusiontooling and reduces cure tooling), and reduces wasted material (drycomponents are infused at time of fabrication rather than when initiallymanufactured for which they are given an expiration date) which reducesthe overall cost of production for a component.

A consolidated infusion and cure cycle may be accomplished using, forexample, resin film and film adhesive in addition to a dry fibercomponent, such as a three-dimensional woven fiberglass pi-preform, andpre-cured composite stiffeners, such as carbon-fiber stiffeners. Toachieve a desired resin content by weight, different resin film weightsmay be applied along certain faces of the pi-preform. Moreover, filmadhesive may be applied to the portion of the carbon-fiber stiffenerresiding inside the clevis of the pi-preform to aid in adhesion/bondingto one and other during cure. Opposed to a film adhesive, whichtypically has a carrier lattice imbedded in the adhesive sheet, allowingit to act slightly more like a fabric, a resin film need not have anembedded carrier and is a free-form sheet of a tacky semi-fluid resinfilm.

Fabrication of the stiffeners, both blade (i.e., substantially flat) andother configurations, such as those depicted in FIGS. 2 a through 2 g,may be fabricated using shaping tools, molds and pre-preg compositelayup techniques known to those of ordinary skill in the art. Thestiffener used to provide support for the infusion, cure, or bonding ofadditional components should not be limited to composite materials suchas carbon-fiber, but could be manufactured from any material thatprovides the required stiffness to the assembly at the time of infuse,cure, and/or bond. The dry fiber component, including three-dimensionalwoven fiberglass pi-preforms, may be fabricated from, for example,S-Glass and infused with resin. S-Glass is generally used for polymermatrix composites that require improved mechanical properties comparedto E-glass-based composites. This is often the case when the material isoperated under more extreme conditions. FIG. 1 provides across-sectional view of an example pi-preform 102 and L-shaped stiffener104 sandwiched between two mandrel halves 106 a, 106 b. U.S. PatentPublication No. 2009/0247034 to Goering discloses additional examplepi-preforms.

A consolidated infusion and cure cycle enables users to produce infusedand fully-cured pi-preforms on an as-needed basis. This is advantageousbecause, while the resin film itself has a finite lifespan and willeventually expire, it is a small fraction of the cost of dry fibercomponents, such as three-dimensional woven pi-preforms. By infusing thepi-preforms as needed, remaining dry fiber components may be storedindefinitely until needed and only require an in-date (i.e., notexpired) batch of resin film at the time of cure. This consolidatedinfusion and cure cycle may also be applied to other pre-manufactureddry composite materials of any number of shapes.

To provide an overview, the present invention may be illustrated by thefollowing example, which is provided to aid in the understanding of theinvention and is not to be construed as a limitation thereof.

Example 1

FIGS. 2 a through 2 i illustrate a first example consolidated dry fibercomponent infusion and cure process using a composite stiffener.Specifically, as illustrated, a pre-cured L-shaped stiffener 202 may befused with a dry fiber component, such as pi-preform 208. FIG. 2 aillustrates a first step wherein the bottom edge portion (e.g., about0.5 inches) of a pre-cured stiffener 202 is wrapped with a film adhesive204. FIG. 2 b illustrates a second step wherein a resin film 206 iswrapped around the bottom edge portion of the pre-cured stiffener 202(i.e., on the film adhesive 204) to form a stiffener subassembly 232.The film adhesive 204 can function to create a superior bond between thepi-preform 208 and the stiffener 202. FIG. 2 c illustrates a third stepwherein the wrapped edge of the stiffener subassembly 232 is insertedinto an open clevis 234 of a pi-preform 208.

FIG. 2 d illustrates a fourth step wherein a resin film 210 is appliedto the top and bottom of the pi-preform 208's base. The weight of theresin film 210 may be adjusted to achieve a desired resin weight contentof the final cured product. FIG. 2 e illustrates a fifth step wherein asecond resin film 212 is applied to the exterior of the pi-preform 208'sclevis. As illustrated, the first and second resin films 210, 212, whichmay be of different weights, are added to the various faces of thepi-preform 208. Specifically, the resin film 212 may be applied to thevertical faces of the pi-preform 208 while the resin film 210 is appliedto all horizontal faces. Depending on the application, the various filmweights may be tailored to provide a specific percent resin weight ofthe final cured composite. Accordingly, this specific percent resinweight may vary from application to application, but may fall within,for example, the 30-40% resin content weight range. To ensure that thecomplete pi-preform 208 is fully saturated with resin, and to aid inease of fabrication, resin film may be overlapped in each of theexterior corners where the clevis meets the base of the pi-preformduring dry pi-preform layup.

FIG. 2 f illustrates a sixth step wherein a peel ply 214 is applied tothe pi-preform 208's base. The peel ply 214 provides a prepared bondingsurface during later manufacturing. It may be preferable to cut the peelply 214 larger (e.g., about 0.25 inches larger) than the base of thepi-preform 208, thereby aiding in applying the peel ply by requiringdecreased accuracy while still covering the intended area. However,superfluous peel ply should be minimized as it can absorb additionalresin, thus affecting the final resin percent by weight of the curedproduct. While a pi-preform 208 is illustrated, depending on theapplication, the stiffener assembly 232 may be inserted in, or bondedwith, other dry fiber components and therefore should not be limited tothe illustrated pi-preform type.

The rigidity of the stiffeners 202 may be increased by implementinggeometric features (e.g., “L”, “J”, “Hat”, etc.) or increasing thethickness of the stiffeners. An advantage of employing a L-shapedstiffener 202, or other geometric shape, is that tooling is notrequired. Specially, additional tooling, which is often used to providestiffness/straightness along the stiffener's length, may be omitted,thereby eliminating any associated set-up time and expense. Mandrels maybe used to perpendicularly orient the stiffener to the dry pi-preformwhile providing uniform pressure during infusion/cure but may not berequired. For example, as FIG. 2 i illustrates, a mandrel half 222 maybe configured on each side of the pi-preform 208's clevis. The mandrels222, may be used to apply pressure to the pi-preform stiffener assemblyto ensure resin flows into the crevices and corners of the pi-preform208 during infusion. The mandrels 222 may be used for infusion, cure,and bonding of the pi-preform components and bonded joints. Thesemandrels can assist in providing additional support for the compositestructures as well as applying uniform pressure. The mandrels 222 mayalso be used to apply uniform pressure along the faces of the pi-preformand into the corners. To prevent unwanted adhesion, the stiffeners 202may be covered in a release film between the laminates and the mandrels222.

FIG. 2 h illustrates a cut-away side view of a final assembly. FIG. 2 iillustrates a pi-preform stiffener assembly in a traditional vacuumbagging assembly 224. For example, the vacuum bagging assembly 224 maycomprise a plurality of vacuum ports 226, a reader port 228, and abreather fabric 230, which may be laid down around the pi-preformstiffener assembly as well as with paths to the vacuum ports 226. Thefinal assembly may then be bagged and put under vacuum to apply pressureduring the infusion/cure processes. The vacuum bag may then be placedinto an oven which heats the material, causing the resin in the resinfilm and/or pre-preg to change from sticky and soft to hard and stiff.Providing pleats in the bag can enable uniform distribution of thevacuum pressure along the mandrels 222 and into the pi-preform stiffenerassembly. The use of breather fabric 230 around the perimeter of thepi-preform stiffener assembly allows air pathways to the vacuum ports226 to create as much pressure as possible on and around the partwithout allowing the back to choke off sections of the layup.

Depending on the size and shape of the manufactured component, the curecycle may be tuned to allow full infusion of the resin prior to cure.Specifically, the oven temperature and time for infusion may be set to aspecific temperature at which point the resin retains a fluid stateallowing it to saturate the dry material. This temperature may differdepending on the resin/epoxy matrix being used; and the time requiredfor it to fully permeate the dry fibers may also vary. In one case,where the resin may cure around 250 degrees, the infusion temperaturemay be around 175 degrees. Where the cure time may be 250-360 minutesthe infusion time may be 10-25 minutes. The stiffeners 202 may be curedon flat plate aluminum to provide a smooth, flat base to the pi-preform,however, due to the benefit of the use of dry fiber components beinginfused during the curing, this process can be employed on any type ofsurface of varying curvature (for example the skins and leading edge ofthe interior of a wing). Once the component is fully infused andsaturated with the resin matrix, the temperature can then be increasedto a point at which curing will occur. Once cured, the oven may bedecreased to ambient temperature. Therefore, an advantage of using themethods described herein is that the composite assembly may essentiallybe simultaneously infused and cured. That is, unlike prior methods, thecomposite assembly may go from infusion to cure by simply increasing thetemperature of the oven, thus eliminating unnecessary tooling and thecosts usually associated with the transition from infusion to cure.

Example 2

The process of Example 2 is substantially the same as the process ofExample 1. However, in certain situations stiffeners 302, such as theblade-shaped stiffener illustrated in FIG. 3 a, may be employed in lieuof stiffeners having geometric shapes. Unfortunately, such blade-shapedstiffeners may not be sufficiently stiff to keep the stiffener structurestraight along the length of the assembly. For example, where ablade-shaped stiffener is too thin and/or must span a greater distance.Thus, unlike Example 1, tooling angles 316 may be further employed tokeep the blade-shaped stiffeners 302 vertical while the lengthwiseclamps 318, which may be attached (e.g., bolted) to the angles 316, cankeep the whole assembly straight along its length. Specifically, asillustrated in FIGS. 3 c and 3 d, a clamp 318 may be configured to spanthe length of the to-be-cured components to keep the blade-shapedstiffener 302 substantially straight along its length. The varioustooling may be fabricated from materials known in the art, such asaluminum. For example, FIG. 3 c illustrates a step wherein toolingangles 316 are used to clamp the ends of the overhanging stiffener 302to keep it vertically straight. In other words, tooling angles 316 maybe used to position the blade-shaped stiffener 302 vertically inside itsclevis 234, keeping it substantially perpendicular to the pi-preform208's base. As with Example 1, the size, shape and length of theblade-shaped stiffener 302 may be increased or decreased to fulfill aparticular application.

While FIGS. 2 a through 2 i and 3 a through 3 f illustrate two examplestiffener shapes (i.e., “L” 202 and “Blade” 302), the above-describedmethods may be applied to stiffeners of various shapes, forms and sizes.For example, FIGS. 4 a through 4 h provide a plurality ofcross-sectional views of example stiffener shapes and configurations.Specifically, FIG. 4 a illustrates the above-mentioned blade-shapedstiffener. FIGS. 4 b and 4 c illustrate J-shaped and L-shapedstiffeners, respectively. As noted above, by increasing geometry (e.g.,providing additional bends or curves in the stiffener), rigidity can beincreased thereby eliminating the need to use additional tooling.Additionally, while the blade-shaped, the L-shaped and the J-shapedstiffeners of FIGS. 4 a through 4 c have a single contact point and thususe only a single pi-preform 208, additional pi-preforms 208 may beimplemented to facilitate additional shapes having multiple contactpoints, thus further increasing strength and/or rigidity. For example,the stiffener may be bent into a narrow or wide U-shape and connected toa substrate using two pi-preforms 208. This is exemplified in FIGS. 4 d,4 e, 4 f, 4 g and 4 h, which illustrate hat-shape, wide hat-shape,hemispherical-shape, vertical bracket and a corner bracketconfigurations, respectively. In addition, as illustrated in FIGS. 4 gand 4 h, a stiffener may be configured to connect parallel orperpendicular surfaces, which may be advantageous in applications suchas airfoil fabrication.

One or more pi-preform stiffener assemblies may be used to providerigidity and strength to composite components, such as those fabricatedfrom GLARE. Thus, pi-preform stiffener assemblies may be bonded to abonding surface using paste adhesive. To enhance bond strength, acomposite material bonding surface may be scuffed using an abrasive pad,and then wiped with, for example, acetone. Once prepared, the compositematerial bonding surface may sit for a period of time (e.g., about onehour) prior to bonding to ensure that, for example, all fluid has fullyevaporated. Any peel ply applied to the base (i.e., underside) of thepi-preform stiffener assembly should be removed just before bonding,thus preserving the surface until bonding. When bonding the components,paste adhesive should be mixed thoroughly and applied to each componentof the bond (e.g., the bonding surface and the pi-preform stiffenerassembly). The GLARE surface may be wetted out with a very thin layer ofadhesive, while adhesive may simultaneously be applied to the base ofthe pi-preform stiffener. An adhesive spreader may be used to uniformlyspread the adhesive. Glass beads premixed into the adhesive help tocontrol bond line thickness. The silicone mandrels used during cure ofthe stiffeners may also be used to apply uniform pressure during thebond. The pi-preform stiffener assembly and GLARE bonding surface may beleft to cure at room temperature under vacuum (typically at 25-27 in Hg)for up to 24 hours. Heat may be applied in some cases to acceleratecuring of the adhesive.

During the curing process, mandrels, like those used during cure of thepi-preform stiffeners, may be used to apply uniform pressure from thevacuum. The only addition to this procedure may be to add flash tapeplaced approximately 0.25″ from the edge of the pi-preform base. Thisflash tape could later be removed once the adhesive cured; taking withit any additional squeeze-out from the bond leaving a precise edge alongthe length of the bonded parts, while still allowing the paste adhesiveto feather from the discrete edge of the pi-preform stiffener down tothe GLARE.

FIG. 5 illustrates an example unitized hybrid structure wherein one ormore pi-preform stiffener assemblies 502 are used to provide rigidityand strength to a composite hatch of the Aircraft's access panel 500.The composite hatch panel 500 may comprise a composite material such as,for example, GLARE. To stiffen the panel, pi-preform stiffenerassemblies 502, which may be approximately 1.0″ wide by 0.5″ tall with afour-ply L-shaped stiffener, may be bonded spanning the hatch panel. Asone of skill in the art would recognize, the size of the pi-preformstiffener assemblies 502 may be adjusted for a particular purpose. TheL-shaped stiffener may be made from, for example, four-ply (45/0/0/45)cloth carbon-fiber pre-preg.

FIGS. 6 a through 6 c illustrate a second example unitized hybridstructure 600. More specifically, FIGS. 6 a through 6 c illustrate anairfoil 600 having a GLARE substrate 602 to which other components arebonded to form a hybrid unitized structure. For example, a stiffener 604may be bonded with the GLARE substrate 602 using a pi-preform 606. Forthis type of application, the stiffener 604 may be substantially planarwith a center portion removed to reduce weight and enable wires, cablesand the like to be run along the length of the component. However, toincrease strength, the stiffener 604 may be fabricated one or morediagonal straight portions, thereby forming a truss. The stiffener 604may be fabricated using techniques known in the art to achieve a desiredshape or size.

FIGS. 7 a and 7 b illustrate a third example of a unitized hybridstructure 700. More specifically, FIGS. 7 a and 7 b illustrate a secondairfoil 700 having a GLARE substrate 702 to which other components arebonded to form hybrid a unitized structure. For example, a stiffener 704with a 708 core may be bonded with the GLARE substrate 602 using api-preform 606. Like the stiffener 604 of FIGS. 6 a through 6 c, thestiffener 704 is substantially planar. However, to provide furtherstiffness, the stiffener 704 may comprise a core 708. Example cores mayinclude, for example, hexagonal-celled core of various materials or foamcore. Specifically, the core 708 can provide additional rigidity duringinfusion, cure and bond, as well as additional strength and stiffness tothe final assembly.

While the present invention has been described with respect to what arepresently considered to be the preferred embodiments, it is to beunderstood that the invention is not limited to the disclosedembodiments. To the contrary, the invention is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims. The scope of the following claims is to beaccorded the broadest interpretation so as to encompass all suchmodifications and equivalent structures and functions.

All United States and foreign patent documents, all articles, brochures,and all other published documents discussed above are herebyincorporated by reference into the Detailed Description of the PreferredEmbodiment.

What is claimed is:
 1. A method of fabricating a composite structurefrom components of varying stages of cure, comprising the steps of:providing a stiffener component; providing a dry fabric component;combining the stiffener component and the dry fabric component to forman assembly, wherein the stiffener component provides structural supportto the assembly; infusing the assembly with resin to yield an infusedassembly; and curing the infused assembly to yield a cured assembly. 2.The method of claim 1, further comprising the step of bonding the curedassembly with a component to form a hybrid unitized structure.
 3. Themethod of claim 1, wherein the stiffener component is fabricated from acomposite material.
 4. The method of claim 1, wherein the stiffenercomponent is (i) L-shaped or (ii) J-shaped
 5. The method of claim 1,wherein the dry fabric component is a three-dimensional wovenpi-preform, wherein said pi-preform contains a clevis configured toreceive an edge of the stiffener component.
 6. The method of claim 3,wherein the stiffener component is fabricated from a carbon-fibercomposite material.
 7. The method of claim 3, wherein the stiffenercomponent is fabricated from a glass-fiber composite material.
 8. Themethod of claim 1, wherein the stiffener component is fabricated from ametal.
 9. The method of claim 5, further comprising the steps of: (i)wrapping a resin film around a first edge of said stiffener component;and (ii) inserting the first edge of said stiffener component into theclevis of said pi-preform.
 10. The method of claim 9, further comprisingthe steps of: (i) wrapping a resin film around a second edge of saidstiffener component; and (ii) inserting the second edge of saidstiffener component into the clevis of the second pi-preform.
 11. Themethod of claim 2, wherein the component comprises Glass LaminateAluminum Reinforced Epoxy.
 12. The method of claim 1, wherein the dryfabric component comprises at least one of: (1) glass-fiber; (2)carbon-fiber; or (3) para-aramid synthetic fiber.
 13. A method offabricating a composite structure from components of varying stages ofcure, comprising the steps of: providing a pre-cured compositestiffener; providing a dry fabric component having a base portion and aclevis configured to receive the pre-cured composite stiffener, whereinthe clevis is substantially perpendicular to the base portion; wrappinga first film adhesive around a first edge of said pre-cured stiffener;wrapping a first resin film around the first edge of said pre-curedstiffener; inserting the first edge of said pre-cured compositestiffener into the clevis of the dry fabric component to form acomposite structure assembly; applying a second film adhesive to asurface of the dry fabric component; applying a second resin film to thebase portion of the dry fabric component; securing the pre-curedstiffener substantially perpendicular to the base portion of the dryfabric component, wherein the pre-cured stiffener provides structuralsupport to the composite structure assembly; applying a film adhesive tothe exterior base portion of the dry fabric component securing thepre-cured stiffener with dry fabric component to another substrate towhich it will bond; infusing the composite structure assembly by heatingthe assembly to a first temperature for a first period of time; andcuring the composite structure assembly by increasing the heat to asecond temperature, wherein the second temperature is greater than thefirst temperature.
 14. A method of fabricating a composite structurefrom components of varying stages of cure, comprising the steps of:providing a pre-cured composite stiffener; providing a dry fabriccomponent having a base portion and a clevis configured to receive thepre-cured composite stiffener, wherein the clevis is substantiallyperpendicular to the base portion; wrapping a first resin film around afirst edge of said pre-cured stiffener; inserting the first edge of saidpre-cured composite stiffener into the clevis of the dry fabriccomponent to form a composite structure assembly; applying a secondresin film to a surface of the dry fabric component; securing thepre-cured stiffener substantially perpendicular to the base portion ofthe dry fabric component, wherein the pre-cured stiffener providesstructural support to the composite structure assembly; infusing thecomposite structure assembly; and curing the composite structureassembly.
 15. The method of claim 14, further comprising the steps ofbonding composite structure assembly with a component to form a hybridunitized structure.
 16. The method of claim 14, wherein the dry fabriccomponent comprises at least one of: (1) glass-fiber; (2) carbon-fiber;or (3) para-aramid synthetic fiber.
 17. The method of claim 14, furthercomprising the steps of: (i) wrapping a resin film around a second edgeof said stiffener component; and (ii) inserting the second edge of saidstiffener component into the clevis of the second pi-preform.
 18. Themethod of claim 15, wherein the component comprises Glass Laminate 19.The method of claim 14, wherein the second resin film has a weight thatis different than the third resin film.
 20. A method of fabricating acomposite structure from components of varying stages of cure,comprising the steps of: providing a pre-cured composite stiffener;providing a dry fabric component having a base portion and a clevisconfigured to receive the pre-cured composite stiffener, wherein theclevis is substantially perpendicular to the base portion; wrapping afirst resin film around a first edge of said pre-cured stiffener;inserting the first edge of said pre-cured composite stiffener into theclevis of the dry fabric component to form a composite structureassembly; applying a second resin film to a surface of the dry fabriccomponent; securing the pre-cured stiffener substantially perpendicularto the base portion of the dry fabric component, wherein the pre-curedstiffener provides structural support to the composite structureassembly; infusing the composite structure assembly by heating theassembly to a first temperature for a first period of time; and curingthe composite structure assembly by increasing the heat to a secondtemperature, wherein the second temperature is greater than the firsttemperature.
 21. The method of claim 20, wherein (i) the firsttemperature is between 150 and 200 degrees Fahrenheit, (ii) the secondtemperature is between 200 and 300 degrees Fahrenheit, (iii) the firstperiod of time is between 10 and 25 minutes, and (iv) the second periodof time is between 250 and 350 minutes.